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There are two methods of oil mist detection, Obscuration and Light Scatter. The earlier forms of oil mist detector used an obscuration type detector, the most well known type being the early Graviners. Schaller use obscuration in their Visatron range of OMDs. Light scatter is a modern method of oil mist detection used by QMI and the Latest Graviner Mk6OBSCURATIONThe older type of obscuration type of detector consists of two parallel tubes of equal size, each having a photoelectric cell at one end which generates an electric current directly proportional to the intensity of the light falling on its surface. Lenses are fitted to seal the ends of each tube but allow light to pass. Two identical beams of light from a common lamp are reflected by mirrors to pass along the tubes onto the cells which are then in electrical balance.The samples drawn from the crankcase are drawn in turn along the measuring tube by means of a selector valve. If a concentr…

FLAME ARRESTORS AND SHIELDINGThe intention of the requirement to shield the valve discharge was to reduce the possible danger to personnel from flame emission. However explosion testing has shown that whilst a flame arrestor will work satisfactorily when shielding is not fitted, when shielding is fitted, the energy from the discharge is focussed in one direction, and there will be an emission of flames during an explosion. The fitting of shielding also reduces the effective outflow area of a valve.Since July 2002 it has been a Lloyds Register rule requirement to fit flame arrestors and to test the relief valve with any proposed shielding to be fitted to the valve when installed on the engine.A test procedure for new crankcase relief valves has been developed over the past four years by MAN B&W, The Physical Test Institute in the CzechRepublic, Hoerbiger, and various classification societies. It has been proposed that this procedure be adopted by all classifi…